1. Field of the Invention
The present invention relates to a piston-type hydraulic fluid reservoir (or working liquid reservoir) which can store a working liquid of a hydraulic circuit, and more particularly to a reservoir technique which forms a hydraulic chamber into a donut shape thus making the movement of a piston smoother.
2. Description of the Prior Art
A known vehicle-use brake hydraulic control circuit such as an anti-lock brake device includes a working liquid reservoir which temporarily stores a working liquid released from a wheel cylinder. Although a capacity of the working liquid reservoir is small, the working liquid flows into or flows out from working liquid reservoir each time a brake control is performed. The working liquid reservoir is, in general, of a piston type, wherein a piston defines a hydraulic chamber and a gas chamber in the inside of a cylinder bore. The movement of the piston is relatively frequent and hence, it is important to effectively guide the piston.
There has been known a technique which guides the piston at two points (that is, two portions). This two-point guide technique can surely perform the guiding compared to the general one-point guiding and also can shorten an axial length of the piston and hence, the two-point guide technique is effective in the miniaturization of the working liquid reservoir.
Japanese Utility Model Laid-open Publication Hei5(1993)-65730 describes a technique in which a hydraulic chamber is defined by a piston head, a guide shaft having a small diameter is mounted on a side of the piston head which is positioned in a gas chamber which is positioned opposite to the hydraulic chamber, and the small-diameter guide shaft is made to penetrate a plug member (a member which closes the gas chamber). To focus an attention to the movement of the piston head, the movement of the piston head is guided at two portions, that is, by an outer peripheral portion of the large-sized piston head per se and an outer peripheral portion of the small-sized guide shaft which is integrally formed with the piston head.
The guiding of the piston side at two portions as described in patent document 1 is basically effective in guiding the piston smoothly. However, the hydraulic chamber defined by the piston maintains the same cylindrical type as in the case of one-point guide and hence, an opening of a passage through which a working liquid flows into or flows out from the cylindrical type hydraulic chamber is also deviated or offset with respect to the hydraulic chamber. Accordingly, the flow of the working liquid which enters or is discharged from the hydraulic chamber is disturbed and hence, there exists a possibility that a force which impedes the smooth movement of the piston is generated although an amount of the force may be considerably small.